The invention relates to radiography and particularly to equalized radiography which improves diagnostic capabilities by selectively subjecting denser parts of the examined object to higher intensity radiation to render them more fully exposed and image them with greater contrast and detail. In such radiography, a fan of penetrating radiation sweeps the object while being locally modulated to vary the radiation intensity both as between different sectors of the fan and in the scanning direction in order to selectively equalize the radiation delivered to the image plane.
In conventional radiography, image quality and diagnostic value can be compromised when the object density differs too much as between different parts of the object. For example, in conventional chest x-rays the mediastinum and retrocardiac area could be underexposed, detracting from the diagnostic value of the image. Equalization radiography, which is capable of varying the local x-ray exposure to areas in the image to compensate for the local patient attenuation, particularly in the case of chest x-rays, can improve image quality and diagnostic value.
Typical examples of such feedback-controlled equalization radiography are discussed in the commonly assigned Wang European Patent Application No. 86308224.4 (based on a U.S. application leading to U.S. Pat. No. 4,953,189; see, in particular, FIG. 8 showing wedge-shaped attenuator elements) as well as in Plewes U.S. Pat. No. 4,773,087 and U.S. Pat. application Ser. No. 07/242,644 filed Sep. 13, 1988 (see, in particular, FIGS. 3 and 5 showing overlapping shutter pins in two rows). Further examples are discussed in U.S. Pat. Nos. 4,675,893, 4,715,056, 4,677,652, and 4,741,012. All of the prior documents cited in this paragraph are hereby incorporated by reference in this specification. Further background material concerning the subject can be found in the documents made of record in said Plewes application.
In a typical prior art example, an x-ray fan beam scans the patient and a modulator unit locally controls the x-rays before they reach the patient in order to modulate the radiation differently as between different sectors of the fan and as between different stages of the scanning movement. The degree and kind of local modulation are under the control of a feedback circuit which locally measures the x-rays in the fan exiting the object. The goal of this local, time varying modulation is to equalize the image, i.e., to reduce the difference in exposure as between different areas of the image. The modulator unit can use a row of modulators or shutters which are individually and selectively movable into the fan to modulate it locally, e.g., by varying the local attenuation, the local beam cross-section, and/or the local exposure time of the x-rays impinging on the object being examined.
While such prior art systems could provide significant improvement, they also could introduce certain types of image artifacts. One type is in the direction of scan and could appear as streaks in the image. The inventors believe that this type of an artifact could be caused by ineffective smoothing between adjacent elements of the modulator unit, especially when at any one time during the scan the settings or positions of these adjacent elements are very different, and that artifacts could also be caused by limitations in parameters such as the sizes of the focal spot, the attenuator elements and the collimator slit at the modulator and the geometry of the modulation arrangement. Such artifacts are also noted in Vlasbloem, et al., RADIOLOGY, Vol. 169, pages 29-34 (Oct. 1988). See, also, Plewes, D.B. and Vogelstein, E., Exposure Artifacts in Raster Scanned Equalization Radiography, Med. Phys. Vol. 11. pp. 158-165 (1984). Other artifacts could be in the direction normal to the scan direction and others could be in other directions or positions. Of course, it is desirable to minimize any deleterious effects of such artifacts on the diagnostic value of the image.